The jackets of virtually all diskettes are polyvinylchloride film containing carbon black for opacity. The film is embossed to create a matte finish for aesthetic purposes, mainly so that scratches and smudges are less noticeable. Furthermore, the matte finish reduces friction between adjacent sheets of film when stacked. Thus the bottom sheet is more readily fed from the stack into equipment for forming a jacket from the sheets.
If a diskette is exposed to heat, the distortion of the vinyl film may cause the jacket to warp into a "pillow" shape which may prevent inserting it into a diskette drive and cause other operational problems. ANSI Specification No. X3B8/78-145 includes a "pillow" test wherein a diskette is heated in an oven for a period of time, allowed to cool to room temperature, and then dropped under its own weight through a slot of a certain thickness. After heating for 72 hours at 52.degree. C., an appreciable proportion of diskettes now on the market will fail to drop through the slot. To pass this test, a few diskette jackets are made of modified vinyl or other films which have better thermal stability, but at significantly increased cost of raw material.
Although it has long been known that biaxially-oriented polyester film such as biaxially-oriented polyethylene terephthalate film would provide better thermal stability, we are not aware of any diskettes on the market that have polyester jackets.